This invention relates to a magnetic sector mass spectrometer that is capable of simultaneously detecting two or more mass dispersed ion beams, and which is particularly useful for the determination of isotopic composition of elements of low atomic mass, for example hydrogen, carbon and oxygen.
The accurate determination of isotopic composition by mass spectrometry is usually carried out by means of a magnetic sector mass analyzer that has a plurality of collectors disposed along its mass-dispersed focal plane. In such a spectrometer each collector is positioned to receive only ions of a given mass-to-charge ratio and is provided with means for reading out the number of ions which it receives during a given time period. Consequently, the ratio of the signals generated by the arrival of several ion beams of different mass-to-charge ratio is unaffected by variations in parameters such as the sample flow rate into the ionization source and the ion source efficiency which affect both beams equally, so that, for example, the isotopic composition of an element in a sample can be determined very accurately. An example of a conventional multi-collector array for a magnetic sector mass spectrometer is given by Stacey, et. al. in Int. J. Mass Spectrom. and Ion Phys. 1981 vol 39 pp 167-180.
In the case when an isotope is present only in a small proportion relative to another isotope having an adjacent mass-to-charge ratio, the property known as abundance sensitivity of the mass spectrometer becomes critically important. Abundance sensitivity is a measure of an interfering signal at any given mass-to-charge ratio M due to the presence of a larger signal at M.+-.1. Unless special precautions are taken the larger peak typically has a "tail", usually greatest on the low mass side of the peak, which often extends to adjacent masses and causes an uncertainty in the true zero of the signal at that mass.
A major cause of the low mass tail is thought to be scattering of the ions composed in the major peak due to collisions with neutral gas molecules in the spectrometer housing. Typically these collisions result in a loss in energy so that the ions that have undergone them appear on the low mass side of the true position on the mass-to-charge axis of a resultant spectrum.
Various arrangements are known to improve the abundance sensitivity of a spectrometer. Firstly, the ion optical arrangements of the analyzer, such as the magnetic sector angle, poleface inclination and curvature and the positions and sizes of the entrance and exit slits can be selected to produce high dispersion to minimise the overlap at the detector between beams comprising ions which differ in mass-to-charge ratio of 1 unit. Examples of this approach include Wollnik, Int. J. Mass Spectrom. and Ion Phys. 1979 vol 30 pp 137-154, Prosser, Int J. Mass Spectrom. and Ion Proc. 1993 vol 125 (2-3) pp 241-266 and Prosser and Scrimegour, Anal. Chem. 1995 vol 67 pp 1992-1997. This approach can be successfully adopted with a simultaneous collection spectrometer, but increasing mass dispersion does not necessarily improve the abundance sensitivity as it may merely result in the centroids of adjacent mass peaks being spaced further apart while the width of the peaks is correspondingly increased. An alternative approach is to provide an electrostatic lens or retarding electrode arrangement between the exit aperture of the analyzer and the detector itself. This electrode may be biased so that it provides a potential barrier which ions must surmount to reach the detector. If correctly set, ions which have lost energy and which are therefore comprised in the unwanted low mass tail of a peak will have insufficient energy to surmount the barrier and will be prevented from reaching the detector. Such devices are taught by Kaiser and Stevens, Report No ANL-7393 of Argonne National Laboratory (Pub. November 1997), Merrill, Collins and Peterson, 27.sup.th An. Confr. on Mass Spectrometry and Allied Topics, June 1979, Seattle, pp 334, Freeman, Daly and Powell in Rev. Sci. Instrum. 1967 vol 38 (7) pp 945-948. This method has not typically been applied to simultaneous collection mass analyzers because the retardation of the wanted ions as they surmount the potential barrier amplifies the relative contribution of any component of velocity they may have perpendicular to their direction of travel and can actually result in a greater overlap between adjacent mass peaks.
An improvement on the provision of a retarding electrode is the use of an energy analyzing device between a magnetic sector analyzer and the ion detector. The three-stage mass spectrometer of White, Rourke and Sheffield described in Appld. Spectroscopy 1958 (2) pp 46-52 comprised two magnetic sector analyzers followed by an electrostatic energy analyzer and was intended to provide improved abundance sensitivity. However, the restriction imposed on the extent of the mass-to-charge focal plane by the final electrostatic analyzer precluded the use of a multicollector detector at this location. Instead, the "low mass" ion beam was deflected into an auxiliary electron multiplier as it left the second magnetic sector and only the high mass ion beam entered the energy analyzer. Thus, when used for its intended purpose of the isotopic analysis of uranium, the .sup.238 U ion beam would pass into the energy analyzer and the .sup.235 U beam would be intercepted after the second magnet. As the .sup.238 U beam was 140.times. more intense than the .sup.235 U beam in the examples given, the presence of the energy analyzer does not prevent .sup.238 U ions which have lost energy striking the .sup.235 U collector because the collector is situated upstream of the energy analyzer. This prior art therefore teaches that an energy filter should be used to filter the most abundant ion beam, but as the authors make clear, when used in the simultaneous collection mode the improvement in abundance sensitivity arises from the presence of the two magnetic sector analyzers and not from the electrostatic analyzer. It is clear that energy filtration of the most intense ion beam subsequent to it passing the collector used for the less abundant beam can have no effect on the interference to the signal at that collector from ions in the most abundant beam that have lost energy.
An isotopic-ratio multicollector spectrometer having a 90.degree. spherical sector energy analyzer is described by Zhang in Nucl. Instrum. and Methods in Phys. Research 1987 vol B26 pp 377-380. This instrument is similar to that described by White, Rourke and Sheffield in that the energy filter is arranged to filter the highest mass ion beam only (ie, the .sup.238 U beam in the example given) while collectors for other ion beams are disposed before the entrance slit of the energy analyzer in such a way that they intercept only lower mass ion beams. Consequently, as in the earlier instrument, if used in a simultaneous collection mode this instrument cannot reduce interference to the less abundant .sup.235 U, .sup.236 U and .sup.237 U beams. The example given suggests that to obtain an improvement in abundance sensitivity the instrument is used in a conventional single-collector mode and the magnetic field is scanned.
U.S. Pat. No. 5,220,167 and International Application WO 97/15944 teach use of an electrostatic lens disposed between the exit of a magnetic sector mass analyzer and an array of collectors in an isotopic ratio mass spectrometer in order to increase the separation between beams of different mass-to-charge ratios at the detector. Such an arrangement does not improve the abundance sensitivity, as explained above.
GB patent application 2230896 teaches the disposition of a retarding lens and a quadrupole mass filter to receive one of the ion beams in a simultaneous collection mass spectrometer to eliminate ions of different mass-to-charge ratios which have lost energy due to scattering from that beam. U.S. Pat. No. 5,545,894 describes a hydrogen isotopic ratio mass spectrometer in which isobaric interferences are reduced by passing ions of hydrogen, deuterium, tritium and helium into a detection device which comprises a thin foil through which the ions must pass. Atomic ions of H, D, and T exit the foil as negative ions and may be separated by scanning an electrostatic energy analyzer disposed downstream of the foil.